The field of this invention relates to rubber compositions containing zinc trimellitate with improved scorch control and an improved control of cure rate.
When sulfur is the vulcanizing agent of a rubber composition, accelerators are used to increase the rate of cure and to improve the physical properties and the age resistance of the vulcanized rubber. With most accelerators small amounts of zinc oxide and fatty acid are required to achieve the best quality in the vulcanized rubber. Hence it is almost universal practice to include both in rubbers vulcanized with sulfur and accelerators. Sometimes further activation and improved properties can be obtained by the use of additional activators such as litharge, magnesium oxide, amines, amine soaps, etc.
Quite often, a single accelerator will give a slow rate of cure but the rate can be increased by the use of secondary accelerators. The combination of accelerator and an activator, such as zinc oxide, to obtain a faster cure rate is more economical than a higher concentration of the accelerator. Use of activators can be accompanied by serious material losses in processing because of "scorch" (premature vulcanization during compounding) wherein the rubbers in process suddenly or gradually become tough and unworkable. This phenomenon described as scorching, precuring or burning may be attributed to (1) faulty compound design, (2) excessive processing temperatures, (3) improper or insufficient cooling before storage between processing stages, or (4) faulty processing equipment. Scorch is one of the rubber industry's major problems. Retarders extend scorch safety but have disadvantages of unpredictable influences on state of cure and undesirable side effects such as staining and porosity.
The prior art discloses a number of vulcanization additives giving rubber products with reduced tendency to scorch. U.S. Pat. No. 2,554,181 teaches cyclohexyldithiocarbamates prepared from cyclohexylamine and carbon disulfide are powerful but nonscorching accelerators for vulcanization of chloroprene rubber. U.S. Pat. No. 2,567,853 teaches the retarding of the vulcanization of rubber to prevent scorch by the addition of the organic hydrogen sulfate such as zinc lauryl sulfate, zinc oleyl sulfate or aluminum oleyl sulfate in the presence of zinc oxide and stearic acid in the vulcanization recipe. U.S. Pat. No. 2,561,524 teaches the use of bicyclo-[2.2.1]-5-heptene-2,3-dicarboxylic anhydride or its derivatives to reduce scorching in the presence of certain accelerators. German Offen. No. 2,403,208 teaches that premature vulcanization of natural rubber was inhibited by thiophosphoramides to increase Mooney Scorch time from 11 minutes to 18 minutes. U.S. Pat. No. 3,850,845 teaches metal paste co-dispersions of magnesium oxide and zinc oxide as curing or vulcanizing agents for halogen-containing polymers such as neoprene. Japanese Kokaii No. 75,128,732 teaches that before curing with metal oxides and thioureas, neoprene rubbers were mixed with cyclohexyl dimethylthiocarbamyl disulfide to inhibit prevulcanization in a composition containing zinc oxide and stearic acid, obtaining a Mooney Scorch test value of 1 minute 55 seconds. U.S. Pat. No. 3,544,531 teaches a method of vulcanizing rubber with dialkyloxyphosphinothioyl amino disulfides as accelerators and vulcanizing agents in typical formulations containing zinc oxide and/or stearic acid.
Accordingly, it is well-known in the prior art to reduce scorching with a number of scorch delay compounds with many different accelerators used with zinc oxide and/or stearic acid. In general, it has been taught in the prior art that any alkaline material, either added as such or present in the compounded rubber for another purpose as is the case of byproduct whitings or high pH furnace blacks, has an activating effect on rubbers in which they are present. Conversely, materials of an acidic nature retard the vulcanizing rate in compounded rubber where they are present. Stearic acid itself, if present in excessive quantities, exerts a retarding action.
Accordingly, it is well-known in the prior art to vulcanize rubber with sulfur and zinc oxide and/or stearic acid in conjunction with many other additives fulfilling functions as accelerators, activators, inhibitors, etc. but vulcanization of rubber with sulfur, zinc oxide and/or stearic acid in conjunction with zinc trimellitate as a scorch control agent and cure rate control agent has not been known.
The new rubber compositions comprising zinc trimellitate and a rubber selected from the group consisting of styrene-butadiene rubber (SBR), nitrile rubber, natural rubber, butyl rubber, polybutadiene rubber and isoprene, the said rubber being sulfur-cured with zinc oxide as an activator, demonstrate not only improved scorch control with increased control of scorch time but also improved control of cure rate.